In the various systems used for air traffic control such as aircraft traffic control and landing system monitors it is required that precise elevation angle measurements be made. Generally, the elevation angle is measured at a ground station which notes the elevation angle from which a signal reflected from the aircraft is received, in the case where radar signals are transmitted from the ground station and reflected from the aircraft. Elevation angle can also be measured at a ground station which notes the elevation angle from which a transponded signal, transmitted from the aircraft in response to an interrogation signal, is received at the ground station. In any event, whenever the elevation angle is small, that is, whenever the aircraft is close to the horizon of the measuring station, ground reflection of the signals received from the near horizon aircraft complicates the elevation angle measurement process. Specifically, signals reflected from the ground into the ground station receiving antenna when considered with the signals received directly from the target aircraft produce a double target return where one target, corresponding to the real target, appears to be above the radar horizon and another target, corresponding to the ground reflected signals, appears to emanate from a below the horizon target. Generally, the returns from these two targets will merge into a single target return centered about the radar horizon and which may have two discernible peaks. Although it would seem that one could determine the true elevation angle by simply taking half the apparent angle between the two target peaks, such an approach will not provide an accurate measure of the elevation angle. This is true for at least two reasons. First, the apparent below horizon target is not usually a true specular target but will include some ground scattering return, or glint, which will distort the resultant dual target return and introduce an angle error. Second, even if the below horizon target were a true specular target, the addition of the specular target return signal to the true target return signal produces a distorted resulting dual target signal because of the side band content of the various target return signals.
Various prior art techniques have been developed to process signals from low elevation aircraft so as to provide accurate elevation angle measurements. These techniques include what have been characterized as the complex angle technique and multitarget estimation techniques. According to the complex angle technique the phase of the sum and difference voltage patterns of the received signal is measured. The measured phase is compared with a predicted set of calibration values which represent the path traced by the vector for a target as it moves through elevation angles up to about one beamwidth above a known surface. Establishing a reliable set of calibration values is a problem when using this approach in certain environments.
Multitarget estimation is the application of the maximum likelihood estimation technique. According to one advanced embodiment of this technique, the ground receiving station calculates amplitude, phase and direction for each target (3 unknowns for each target, 3 N unknowns for N targets) from the receiving station antenna data. These data contain amplitude and phase information of the wave received by each antenna element (2 measured values for each antenna element; 2 M measured values for M antenna elements). Choosing 2 M to be greater than or equal to 3 N, the 3 N unknowns can be determined by known devices. This embodiment works well when the detector noise is very low such as -40 db or lower. The problem with this embodiment is that the high signal to noise ratios implied by such noise figures are seldom if ever attainable in the real world.
Another embodiment of the multitarget estimation technique uses a complex antenna system having such features as time gating (to select one target and reject all other targets except for specular interference), and elimination of elevation angle ambiguity due to the small number of antenna array elements. The influence of specular reflection is reduced as follows. The phase difference of the field received by the end pair of the receiving antenna elements is measured by an interferometer. This phase angle directly corresponds to the elevation angle to be measured. In order to reduce the specular multipath return, a passive spatial filter is introduced which sharply cuts out the signal arriving from a direction below the horizon. The elevation angle measurement accuracy using this embodiment is poor, however, when the target elevation angle is higher than about one antenna beamwidth.
In a proposed third embodiment of this multitarget estimation technique single edge processing is employed. This technique is based on the fact that, as a radar beam is scanned in the elevation plane from high angles to low angles, the leading edge of the beam is not distorted by ground reflections of the main beam. The central location of the beam main lobe, which corresponds to the elevation angle to be measured, is estimated by measuring the beam leading edge location. This technique works well for very high frequency signals (above about 5060.7 MHz) but measurement accuracy deteriorates at lower frequencies.
There are several other techniques for measuring low elevation target angles. However, the ones mentioned above appear to be the best of the prior art techniques.